In the past, I only set the shell variable PATH to make the software compiledby me can be found.

And after I installed TeXLive 2016 I learnt how to set MANPATH, which allows me to use man to browse the manual.

But today, I find what I was doing is absolutely wrong.

I use Linux Mint 18.

the way I set MANPATH was like this

export MANPATH=/path/to/the/man/page:${MANPATH}

Well, that is wrong because in this way, the MANPATH variable is set and the mandb no longer scans the system file /etc/manpath.config, which is relied by all the, how to say, native program.

If mandb runs, it will give a line like this

mandb: warning: $MANPATH set, ignoring /etc/manpath.config

The reason is that, in Ubuntu based system, as I read online, the MANPATH is null by default. Yes, the system will create the database and yes, the man works for all the program like ls and all the program installed via apt. But the system does not set the variable MANPATH.

The right way to set it is like this, as far as I tested

unset MANPATH
export MANPATH=$(manpath -q)
# somthing else
export MANPATH=/path/to/the/man/page:${MANPATH}
# something else

The second line up there is to set the MANPATH the initial value it should be as the /etc/manpath.config is sourced. Notice that the first line is also indispensible 'cause the manpath excutable will ignore /etc/manpath.config as well if MANPATH is set.